Survivors of Canada’s residential schools have expressed dismay after Pope Francis refused to apologize on behalf of the Catholic church for a system that abused thousands of indigenous children for generations.

The schools, many of which were run by missionaries, were used to convert indigenous children to Christianity through a governmental policy of “aggressive assimilation”. More than 150,000 children passed through around 80 schools across the country until the last one closed in 1996.

The Canadian government formally apologized for the program ten years ago. In 2014, Canada’s Truth and Reconciliation Commission recommended a papal apology, which the prime minister, Justin Trudeau, personally lobbied for when he visited the Vatican last year.

While he has apologized for the “grave sins” of colonialism in South America, in a letter released Tuesday by the Canadian Conference of Catholic Bishops, the president of the organization said Pope Francis would not issue a personal apology.